Laadan: Laadan and Libni, seem to have been two distinct names of this person; but the variation of Shimi and Shimei exists only in the translation, the original being uniformly שׁמעי [Strong's H8096]. Exo 6:17, Libni, Shimi

from the age: At first David appointed the Levites to serve from thirty years old and upwards; but considering, probably, that the temple which was about to be built, with its courts, chambers, etc., would require a more numerous ministry, he fixed this period, by this subsequent regulation, at twenty years and upwards. In the time of Moses, the age was from thirty years to fifty. here this latter period is not mentioned, probably because the service was not so laborious now; for the ark being fixed, they had no longer any burdens to carry; and therefore even an old man might continue to serve. See note on Num 8:24.

for all manner of measure: The standards of all weights and measures were in the sanctuary; and therefore the Levites had the inspection of weights and measures of every kind, that no fraud might in this way be committed. Honesty is inseparably connected with piety; and hence the Levites, being sufficiently numerous, were employed to superintend the former, as well as the latter. Lev 19:35, Lev 19:36; Num 3:50